The dehydrogenation of hydrocarbons is an important commercial hydrocarbon conversion process because of the existing and growing demand for dehydrogenated hydrocarbons for the manufacture of various chemical products such as detergents, high octane gasolines, oxygenated gasoline blending components, pharmaceutical products, plastics, synthetic rubbers, and other products which are well known to those skilled in the art. One example of this process is the dehydrogenation of isobutane to produce isobutylene which can be polymerized to provide tackifying agents for adhesives, viscosity-index additives for motor oils, and impact-resistant and antioxidant additives for plastics. Another example of the growing demand for isobutylene is the production of oxygen-containing gasoline blending components which are being mandated by the government in order to reduce air pollution from automotive emissions.
Those skilled in the art of hydrocarbon conversion processing are well versed in the production of olefins by means of catalytic dehydrogenation of paraffinic hydrocarbons. In addition, many patents have issued which teach and discuss the dehydrogenation of hydrocarbons in general. For example, U.S. Pat. No. 4,430,517 (Imai et al) discusses a dehydrogenation process and catalyst for use therein.
Most catalysts for the dehydrogenation of hydrocarbons are susceptible to deactivation over time. Deactivation will typically occur because of an accumulation of deposits that block active pore sites or catalytic sites on the catalyst surface. Where the accumulation of coke deposits causes the deactivation, reconditioning the catalyst to remove coke deposits restores the activity of the catalyst. Coke is normally removed from the catalyst by contact of the coke-containing catalyst with an oxygen-containing gas at a high enough temperature to combust or remove the coke in a regeneration process. In a moving bed process, the regeneration process is carried out by removing catalyst from the vessel in which the hydrocarbon conversion is taking place and transporting the catalyst to a separate regeneration zone for coke removal. Arrangements for continuously or semi-continuously removing catalyst particles from a bed in a reaction zone for coke removal in a regeneration zone are well known. U.S. Pat. No. 3,652,231 describes a continuous catalyst regeneration process which is used in conjunction with the catalytic reforming of hydrocarbons, the teachings of which are hereby incorporated by reference. In the reaction zone of U.S. Pat. No. 3,652,231, the catalyst is transferred under gravity flow by removing catalyst from the bottom of the reaction zone and adding catalyst to the top while reactants flow cross currently through a radial flow bed.
In the past, flow-related phenomena have limited mass flow and fluid velocity through the radial flow beds. One phenomenon, known as “pinning,” inhibits catalyst transfer in many reactor arrangements. Pinning occurs when the flow of fluid at sufficient velocity blocks the downward movement of catalyst. Pinning is a function of the gas composition, the gas velocity, the physical characteristics of the catalyst, and the physical characteristics of the flow channel through which the catalyst must move. As the gas flows through the channels that retain the catalyst, the gas impacts the catalyst particles and raises intergranular friction between the particles. When the vertical component of the frictional forces between the particles overcomes the force of gravity on the particles, the particles become pinned. As the flow path length of gas through the catalyst particles becomes longer, the forces on the particles progressively increase from the outlet to the inlet of the flow channel.
Another flow-limiting phenomena is called “void blowing”. Void blowing occurs when the gas velocity displaces from a surface of the catalyst bed across which it flows from the screen or other retaining elements, thereby creating a void space. The rapid circulation or churning of catalyst over the free surface of the void space can abrade or break catalyst particles, resulting in the production of reduced size particles or fines. This fine material can plug the catalyst bed, exacerbate the void blowing problem, or accumulate in other process piping or equipment in a manner that interferes with the continued effective operation of the process.
As technology has improved and problems such as pinning and void blowing have been better understood, it has been possible to increase the fluid velocity through the radial flow beds of existing reaction zones that were previously limited by these fluid flow phenomena. Increasing the velocity or throughput is of course desirable because it permits an increase in capacity with only minor operating changes to the existing equipment. However, it has been found that further increases in the capacity of many existing dehydrogenation units cannot only be obtained with decreases in conversion or selectivity of the products produced. The limitations in conversion and/or selectivity are related to the reduced time that results from the higher fluid velocity through the catalyst beds. The catalyst contact time is typically expressed in terms of the liquid hourly space velocity (LHSV) of the feed through the catalyst bed. Higher LHSV's may be compensated for, to some extent, by an increased reaction temperature which raises the catalyst activity. Although compensating for higher throughput by increasing the reactor temperatures may maintain conversion levels, it is typically at the expense of lower selectivity and increased coke production on the catalyst.
The most direct way to overcome the problems of space velocity limitations is to add more catalyst to the process. Increasing the catalyst volume is readily accomplished in the design stage for a new unit. Unfortunately for existing units, adding additional catalyst could require expensive modification or replacement of all of the reactors and the associated piping for the delivery of reactants and the transfer of catalyst between the reactors.
Accordingly, it is an objective of this invention to increase the throughput of existing dehydrogenation reactors with only limited modifications to the reaction zone and the associated piping.